1. Field of the Invention
The present invention relates to a method for driving a display device. More specifically, the present invention relates to a method for driving a liquid crystal display device using high-speed write type thin film transistors having a carrier mobility of 1 cm.sup.2 /V.multidot.S or more, and preferably 10 cm.sup.2 /V.multidot.S or more. Hereinafter, such a liquid crystal display device is referred to as a TFT-LCD.
2. Description of the Related Art
FIG. 4 schematically shows the configuration of a TFT-LCD. Referring to FIG. 4, a plurality of pixel electrodes 41 are arranged in a matrix on an insulating substrate 40. A plurality of gate lines 43 and a plurality of data lines 44 are arranged on the insulating substrate 40, running in a row direction and a column direction, respectively, between adjacent pixel electrodes 41. A thin film transistor 42 (hereinafter, referred to as a TFT) is arranged at each crossing of the gate lines 43 and the data lines 44. A drain electrode 42a of the TFT 42 is connected to the corresponding pixel electrode 41. A gate electrode 42b of the TFT 42 is connected to the corresponding gate line 43, while a source electrode 42c thereof is connected to the corresponding data line 44. A counter electrode (not shown) is disposed above the pixel electrodes 41 via a liquid crystal layer so as to oppose the pixel electrodes 41. A voltage is applied between the pixel electrodes and the counter electrode so as to change the orientation of liquid crystal molecules in the liquid crystal layer. By controlling the voltage to be applied between the respective pixel electrodes and the counter electrode, image display is performed by use of a change of the optical characteristic of the liquid crystal layer due to the change of the orientation of the liquid crystal molecules.
A method for driving a TFT-LCD with the above configuration is disclosed, for example, in Japanese Laid-Open Patent Publication No. 60-59389. Such method will be described as follows with reference to FIG. 5.
Pulsing scanning signals 51 are sequentially applied to the gate lines 43, while an image signal 52 is input to the data lines 44 in synchronization with the pulsing of the scanning signals 51. The TFTs 42 are of an n-channel type in this example. Thus, when the scanning signal 51 is at the HIGH level, the channel of each of the corresponding TFTs 42 is activated (ON state), allowing the corresponding pixel electrode 41 and the corresponding data line 44 to be electorically conencted. At the time when the scanning signal 51 becomes the HIGH level, the data line 44 is supplied with a desired image voltage by the image signal 52.
With a current technical trend in TFT-LCDs for improving the resolution of the screen, efforts for reducing the size of TFTs and improving the performance of TFTs have been made. Conventionally, most commercialized TFT-LCDs use amorphous silicon as the semiconductor material. Recently, however, in order to increase the speed of charging loads and improve the resolution of the screen, non-amorphous silicon which has higher crystallinity than the amorphous silicon, such as polysilicon and micro-crystalline silicon, has been used increasingly as the material for a semiconductor layer constituting the TFT. Incidentally, other current technical trends for reducing the size of the TFTs include reducing the gate length and thinning a gate insulating film.
The above trends involving TFT-LCDs leads to the necessity of reducing the voltage applied to the TFTs. More specifically, the use of a semiconductor material having a higher mobility than amorphous silicon, such as polysilicon, is advantageous in that the ON current of the TFT, i.e., the current which flows when the TFT is in the ON state, is large and the load charging speed increases. However, it is disadvantageous in that the OFF current of the TFT, i.e., the leak current which flows when the TFT is in the OFF state, is also large. As shown in FIG. 6, the OFF current decreases as the voltage applied between the source and drain of the TFT decreases. Thus, reducing the voltage applied to the TFT is required.
Reducing the gate length of the TFT and thinning the gate insulating film result in increasing the strength of an electric field applied to the TFT. This causes intrusion of carriers into the insulating film and resultant insulation breakdown. Thus, the reliability of the TFT is lost. These problems arising from the increase in the strength of the electric field can be minimized by reducing the voltage applied between the source and drain of the TFT. Thus, reducing the voltage applied to the TFT is also required from the aspect of the reliability of the TFT.
Further, it is known that the orientation of liquid crystal molecules at the edge portions of the pixel electrodes is disturbed due to a potential difference between the pixel electrodes and the bus lines for the source electrodes and the gate electrodes, i.e., the data lines and the gate lines, causing a defective display. Japanese Laid-Open Patent Publication No. 4-323624, for example, describes this display defect as a prior art problem. This display defect can be overcome by reducing the voltage to be applied between the source and drain of the TFT. Thus, reducing the voltage applied to the TFT is also required from the aspect of the display quality in relation to the liquid crystal.